None of the presently available silicon nitride powders known to me can meet the special chemical purity requirements believed to be necessary for intermediate temperature strength and oxidation resistance while simultaneously meeting microstructural requirements for fabrication into complex shapes. Such characteristics from silicon nitride powders are needed and vital for heat engine performance of sintered silicon nitride and its alloys. I am disclosing in this specification an innovative powder-making process which achieves these ends by being of high chemical purity with respect to certain classes of elements known to be deleterious in sintered silicon nitride ceramics.
Advancement in the area of high temperature mechanical properties for articles made from silicon nitride powder is linked to reduction in the amount of impurities found at the grain boundaries and a reduction of concentration of fluxing impurities in that grain boundary phase. Indeed, the ratios of impurities in the grain boundary and oxide second phases determine not only the eutectic and glass transition temperature but play a key role in oxidation and crystallization of second phases for high temperature mechanical properties. Quantitative studies of oxidation have shown that there is less nitrogen released than oxygen taken up on a stoichiometric basis for certain tested samples of articles made from silicon nitride. This inconsistency points to alkali and alkaline earth impurities being oxidized rather than only the silicon nitride. Intermediate temperature oxidation resistance, in the vicinity of 1000.degree. C., is a major present need for articles manufactured from silicon nitride powders. Chlorine residue appears to interfere with the controlled crystallization of second phases after densification. Because there is so little second phase in such articles, and even less is desired, a much higher purity of silicon nitride powder with respect to these elements is needed than presently available to obtain a modest improvement in the intergranular phase purity. Higher purity with respect to these elements in the silicon nitride powder of desired physical characteristics is essential for advancing the state of the art of grain boundary engineering.
The prior art for synthesis of silicon nitride powders that I am aware of includes several categories, as indicated by the raw material that is the source of the element silicon.
In the well known case for which silicon nitride is derived from nitriding of silicon metal, alkaline earth metal impurities (such as calcium) and transition metal impurities (such as iron) are typically present in the product silicon nitride in quantities that are detrimental to the properties of sintered silicon nitride. Indeed, iron impurity is intentionally added as an aid to the nitriding. Due to the solid particulate nature of the silicon raw material, reduction of impurities (as by leaching) is of limited effectiveness, and the physical characteristics of the starting material are subject to many prior processing variables. The silicon nitride so formed typically has 10% or greater of the beta phase present, which phase is detrimental to the subsequent growth of interlocking, non-equi-axed grains in the ceramic sintering process.
Reaction of high purity silicon-containing gases, such as SiH.sub.4, with ammonia have been used to produce silicon nitride. These processes produce an ultrafine crystallite size (0.03 micrometer and finer) which is not well suited for ceramic forming and suffer excessive shrinkage upon sintering.
Liquid silicon chloride compounds as a raw material have been used in the preparation of silicon nitride. Silicon nitride of high purity with respect to metal-containing compounds, such as calcium and iron, can be synthesized from silicon tetrachloride. However, any chlorine residues from the raw material which remain in the silicon nitride powder are detrimental to grain boundary phase development in a sintered ceramic made therefrom.
The prior art of processes of carbothermal nitriding type that I am aware of includes an article entitled "Preparation of Silicon Nitride From Silica", Chang et al, J. Am. Ceram. Soc., 67 (10), 691-695, 1984. This paper describes the nucleation and growth of silicon nitride from a carbon/silicon dioxide mixture in an nitrogen atmosphere at 1400.degree. C. The study conducted was one in which the specific surface area, particle size, and distribution of silicon dioxide and carbon was varied. The authors noted that yield increased and particle size decreased with increasing silicon dioxide and carbon specific surface area. In this particular paper, the authors discuss the decomposition of tetraethyl orthosilicate (TEOS) by contact with water and utilize the conventional practice of mechanically mixing in carbon black powder produced by commercial suppliers. Of course, the carbon black powder could contain any sort of impurity because it is a powder and is dependent upon the process and the producer who define exactly what is contained therein. The variable physical structure of carbon black and the variable mixing process limit the consistency of the product.
In a similar vein, others have mixed carbon powders with silica (which are products of reaction of TEOS and silicon chloride compounds) and treated the resulting mixture under nitrogen to form silicon nitride. As reported in Chemical Abstracts Nos. 92:200365j and 92:134028y, these citations are to Jpn. Kokai Tokyo Koho Nos. 79,138,898 and 79,138,899, both by Hiroshi Endo et al.
In another carbothermal nitriding process, U.S. Pat. No. 3,855,395 is directed to the production of silicon nitride from rice hulls. The patent discloses the production of silicon nitride from rice hulls where rice hulls are reacted with nitrogen at an elevated temperature either singly or in combination with a catalyst of iron. The rice hulls, by virtue of their biological source, are heavily laden with cation impurities which will affect the purity and quality of the silicon nitride powder made therefrom and the grain boundary phases in sintered silicon nitride which are fabricated therefrom.
U.S. Pat. No. 4,117,095 also discloses a method of making alpha type silicon nitride powder. The alpha silicon nitride powder disclosed is prepared by heating a powdered mixture of silica, carbon, and metallic silicon in a nitrogen-containing atmosphere and then subjecting the material to a heat treatment in an oxidizing atmosphere for decarbonization of that material.
The process of my invention is one which produces an alpha silicon nitride powder from a single liquid precursor for silicon and carbon. A consistent product is produced because of the consistent input materials and consistent process controls. The process is carried out by a high temperature synthesis of an alpha silicon nitride obtained from the reaction of an optionally seeded organometallic precursor, TEOS, with ammonia and an optional hydrocarbon gas or oxidant.
The consistency of the raw material input follows from the use of a liquid feed material, TEOS, that is substantially free, in ordinary form, of chlorine, any other halogen and metals. Chlorine and other halogens have adverse consequences in heat engine applications of silicon nitride ceramics being produced therefrom. Impurities such as halogens can be efficiently removed from liquids to the extent desired because of the many types of processing available for purification of liquids that are not available for particulate raw materials.